This invention relates to solid propellant compositions and to an improved internal plasticizer system for use therewith. In a more particular aspect, this invention concerns itself with a novel branch-chained, unsaturated ester having a particular molecular structure and to its use as an internal plasticizer for isocyanate-cured propellant compositions.
The increased interest and utilization of solid propellant compositions has spawned a considerable research effort in an attempt to improve their ballistic and physical properties. Generaly, solid propellants are composed of one or more organic or inorganic oxidizers dispersed in a resinous binder matrix which may also function as a fuel. Typical oxidizers are ammonium perchlorate or HMX (cyclotetramethylene tetranitramine), both of which are well known in the art. Various resinous components, such as hydracarbons, polyesters, polyurethanes and other like materials may serve as a binder/fuel matrix. A supplemental fuel component, such as finely powered aluminum, may be used also. Other additive components, such as anti-oxidants, burning rate modifiers, wetting agents, anti-foaming agents and plasticizers may be added to the propellant composition, if desired. Dibutylphthalate, dioctyl adipate, or triacetin are often employed as inert plasticizers in combination with the resinous binder material.
Plasticizers are used in rocket propellants for a number of reasons. These include processing assistance by incorportion of fluid materials in the propellant mix, the improvement of low temperature flexibility and the improvement of mechanical properties and ballistic characteristics. In using solid propellants, however, a problem exists in the use of plasticizers because of the tendency of plasticizer molecules to migrate or evaporate from a propellant composition in storage. Either of these processes results in chemical changes in the composition which are harmful to the propellant and to other inert parts of the rocket motor. Also, on long term storage, changes in mechanical properties of the propellant often occur, particularly near the liner-propellant bond. Other negative effects which are associated with the use of plasticizers, include crystallization at low temperatures, a tendency to soften the propellant excessively at high temperatures, and the fact that the plasticizer may migrate into the liner from the uncured propellant much more rapidly than it does from a cured propellant system. Consequently, a continuing research effort has been conducted in an attempt to solve the migration problem associated with propellant plasticizers, and to provide a plasticizer that does not migrate rapidly even in an uncured propellant.
One research effort involved the use of a material known as ZL-496. This material, ZL-496, is a polybutadiene with a molecular weight of approximately 3,000. The use of ZL-496, as a plasticizer, was contemplated because of its molecular size. It is quite large and it was believed that its polymer chain entanglement would prevent migration. Unfortunately, however, it was found that even plasticizer molecules as large as ZL-496 have a tendency to migrate out of the propellant composition and shorten the shelf life of a rocket motor.
In furthering the above research effort, however, it was unexpectedly discovered that a branch-chained, unsaturated ester having a particular molecular structure could be utilized as a plasticizer in the fabrication of solid propellants. This novel plasticizer replaces the conventional plasticizers generally utilized in composite propellants, especially those having a binder base which utilizes an isocyanate cure reaction for its curing system. The resulting propellant exhibits a minimum amount of plasticizer migration during normal storage and exhibits a very good shelf-life.